This invention relates to a pressure sensor whose sensitive element is a semiconductor diaphragm such as of silicon on which is formed a diffused resistor, and more particularly to an improvement in the construction of a pressure sensor of this type mainly with the view of improving the temperature characteristics and stability of performance against physical forces externally exerted on the sensor.
It is known to convert pressure signals into electrical signals by utilizing a piezoresistive effect of a semiconductor crystal. A conventional pressure sensor as a typical embodiment of this method has a silicon plate or block a portion of which takes the form of a diaphragm and a diffused resistor formed in a surface region of the silicon diaphragm. The silicon block is fixedly mounted on a base plate of an electrically insulating material and encased in a housing assembly such that the diffused resistor faces a vacuum chamber defined in the housing, while a fluid pressure subject to measurement can be applied onto the other side of the silicon diaphragm through a pipe attached to the housing and an aperture bored through the plate. The silicon block is bonded hermetically to the base plate which is bonded hermetically to an inner surface of the housing, so that the fluid pressure does not intrude into the vacuum chamber and, hence, does not arrive at the diffused resistor. When there is a difference between the pressures on the two sides of the silicon diaphragm, the resistance of the diffused resistor varies proportionally to the magnitude of the pressure difference or a resultant stress on the silicon diaphragm.
Compared with a more popular pressure sensor of the metal wire strain gauge type, the silicon diaphragm pressure sensor has an exceedingly high sensitivity and accordingly serves for precise measurement of pressure differentials even of very small magnitude. Since the diffusion resistor is always isolated from an external gas atmosphere, this pressure sensor is quite stable even when used in a chemically active gas atmosphere. Recently this pressure sensor has attracted attention in automobile industries as of use, for example, for accurate measurement of the magnitude of a negative pressure created in the intake system for the engine as a primary factor in a precise control of either air-to-fuel ratio or exhaust gas recirculation rate.
From a practical viewpoint, however, the above described silicon diaphragm pressure sensor is still unsatisfactory in its resistance to mechanical shocks and tolerance for rough handling. Since the silicon block is secured to the housing to which is fixed the pressure introduction pipe, the silicon diaphragm is liable to be distorted resulting in that a significant change occurs in the output characteristic of the sensor, when a mechanical force is exerted on the pressure introduction pipe as in the case of the sensor falling on a hard surface.